Method for improving internal center segregation and center porosity of continuously cast strand

Abstract

A method for improving the internal center segregation and center porosity of a continuously cast slab, wherein an unsolidified side edge portion and a given area at the upstream side of the cast slab during continuous casting are defined as a plane reducing zone; a holding means is provided having two sets of top and bottom walking plane reducing compressing means at the plane reducing zone, front and rear supporting shafts common to the sets, eccentric cams for each set arranged at the front and the rear supporting shafts for holding and releasing of the cast slab, and a front and a rear displacement mechanism; the cast slab holding position of the upper surface of the bottom side walking plane reducing means of each set is set within 0.5 mm of the deviation on a passline of a continuous casting machine; the cast slab holding position of the lower surface of the top walking plane reducing means of each set is set at a desired reduction taper having a plane reduction ratio of 0.5 to 5.0% in accordance with an amount of solidified shrinkage of an unsolidified cast slab in a longitudinal compressing plane reducing zone and an amount of the heat shrinkage of the solidified shell; said eccentric cam set and the front and the rear displacement mechanisms are driven to operate the holding, moving forward, opening, and moving backward alternately thereby compressively carrying the cast slab; wherein the improvement comprises the steps of measuring, for each the two sets of plane reducing means the holding distance of the cast slab at before and after the top and the bottom walking plane reducing means, obtaining reduction taper from the measured holding distances and predetermined distances of distance measured positions before and after the top and the bottom walking plane reducing means, obtaining the difference between the reduction taper, then controlling positions of the front and the rear supporting shafts so that each set of walking plane reducing means is given to the desired reduction taper when the obtained difference is 0.1 mm/m or less, and bringing the walking plane reducing means having the measured reduction taper least different from the desired reduction taper close to the other measured reduction taper by changing the plane reduction ratio within a range of 0.5 to 5.0% by controlling the amount of rotation for releasing the holding of the eccentric cams, when the difference is more than 0.1 mm/m and the reduction taper are all less than said desired reduction taper.

We claim: 
     
       1. In a method for improving internal center segregation and center porosity of a continuously cast slab cast from a continuous casting machine having a passline, wherein a plane reducing zone is defined from a solidified portion downstream from an unsolidified end portion of said slab to a selected slab portion upstream from said unsolidified end portion during continuous casting of said slab, said method comprising: providing a slab reducing means for said slab at said plane reducing zone, said slab reducing means having:   a group of top and a group of bottom walking bars,   each group of walking bars composed of a set of outer bars and a set of inner bars disposed between the outer bars defining a walking plane reducing means,   the slab reducing means carrying out an upward and downward movement of the inner and outer walking bars of each set of walking bars to hold, reduce, and release said slab,   said slab reducing means comprising front and rear support shafts common to said sets, eccentric cams for each set arranged on said support shafts, wheel bearings arranged about said eccentric cam, and a rotating mechanism means for rotating said eccentric cams,   said slab reducing means including forward direction and rearward direction displacement mechanism means for forward and rearward displacement of said sets of top and bottom walking bars,   each of said inner and outer bottom walking bars having an upper surface for holding said cast slab, said upper surfaces of a respective set of bottom walking bars being positioned within 0.5 mm deviation from said continuous casting machine passline when said cast slab is being reduced and held by said set of said bars,   each set of said inner and outer top walking bars having a lower surface for holding said cast slab, said lower surfaces of said top walking bars being set at a selected predetermined reduction taper obtained by the amount of solidified shrinkage of the unsolidified slab and heat shrinkage of the solidified shell in the plane reducing zone, said reduction taper being within a plane reduction ratio of 0.5 to 5.0% when said reduction taper is converted into plane reduction ratio,   wherein said slab reducing means including said forward direction and rearward direction displacement mechanism means operates to hold, move forward, release, and move rearward each set of said inner and outer walking bars set to thereby alternately compressively carry the cast slab,   the improvement comprising:   measuring for each of the two opposed sets of walking bars after the start of holding and before release holding distances D 1  and D 2  on the slab for the top and bottom walking bars, D 1  and D 2  corresponding to the thickness of the slab at a spaced apart longitudinal distance D, and obtaining the reduction taper of said top walking bars by the formula:   (D.sub.1 -D.sub.2)/D       comparing the differences between the reduction tapers of each set of top of walking bars, wherein when the difference between the reduction tapers is 0.1 mm/m or less, each of said top walking bars is reduced by said slab reducing means to obtain the predetermined reduction taper after obtaining differences between the measured reduction tapers and the predetermined reduction taper,   and wherein when difference between the reduction tapers is more than 0.1 mm/m and each of said reduction tapers of the top walking bars is less than said predetermined reduction taper, the top walking bars having a smaller different distance from the predetermined reduction taper is positioned by said slab reducing means to obtain the reduction taper of the other top walking bars, after agreement of the reduction tapers of both inner and outer top walking bars, both the inner and outer top walking bars are reduced by said slab reducing means to the predetermined reduction taper.   
     
     
       2. A method according to claim 5, wherein plane reduction is carried out while maintaining the following relationship between the maximum compressive holding width W 0  of the walking plane reducing means in a width direction of the cast strand at the upstream edge (the walking plane reducing means entrance side) in said plane reducing zone and the unsolidified end portion width W of the cast slab;   -60 mm≦W-W.sub.0 ≦200 mm.     
     
     
       3. In a method of improving internal center segregation and center porosity of a continuously cast slab cast from a continuous casting machine having a passline, wherein a plane reducing zone is defined from a solidified portion downstream from an unsolidified end portion of said slab to a selected slab portion upstream from said unsolidified end portion during a continuous casting of said slab, said method comprising: providing a slab reducing means for said slab at said plane reducing zone, said slab reducing means having:   a group of top and a group of bottom walking bars,   each group of walking bars composed of a set of outer bars and a set of inner bars disposed between the outer bars defining a walking plane reducing means,   the slab reducing means carrying out an upward and downward movement of the inner and outer walking bars of each set of walking bars to hold, reduce, and release said slab,   said slab reducing means comprising front and rear support shafts common to said sets at front and rear ends of said sets, eccentric cams for each set arranged on said support shafts, wheel bearings arranged about said eccentric cams, and a rotating mechanism means for rotating said eccentric cams,   said slab reducing means including forward direction and rearward direction displacement mechanism means for forward and rearward displacement of said sets of top and bottom walking bars,   each of said inner and outer bottom walking bars having an upper surface for holding said cast slab, said upper surfaces of a respective set of bottom walking bars being positioned within 0.5 mm deviation from said continuous casting machine passline when said cast slab is being reduced and held by said set of said bars,   each of said inner and outer top walking bars having a lower surface for holding said cast slab, said lower surfaces of said top walking bars being set at a selected predetermined reduction taper obtained by the amount of solidified shrinkage of the unsolidified slab and heat shrinkage of the solidified shell in the plane reducing zone, said reduction taper being within a plane reduction ratio of 0.5 to 5.0% when said reduction taper is converted into plane reduction ratio,   wherein said slab reducing means including said forward direction and rearward direction displacement mechanism means operates to hold, move forward, release, and move rearward each set of said inner and outer walking bars to thereby alternately compressively carry the cast slab,   the improvement comprising:   measuring plane reaction force at the front end and rear end of each set of top and bottom walking bars caused by holding of the cast slab by the walking bars at a selected rotary angle of the eccentric cams and obtaining a first ratio between the measured values of the plane reducing reaction forces at the front end and rear end of each inner set and outer set of the walking bars,   obtaining a second ratio from the measured first ratio and a desired predetermined ratio of the plane reducing reaction forces of each set of the walking bars,   controlling the plane reducing reaction forces while holding the cast slab by said slab reducing means so that said second ratio is from 0.9 to 1.1.   
     
     
       4. A method according to claim 3, wherein plane reduction is carried out while maintaining the following relationship between the maximum compressive holding width W 0  of a plane reducing means in a width direction of the cast strand at the upstream edge (the walking plane reducing means entrance side) in said plane reducing zone and the unsolidified end portion width W of the cast slab:   -60 mm≦W-W.sub.0 ≦200 mm.